This invention relates to a bi-phase phase shift keying (BPSK) demodulator. More particularly, it relates to a BPSK demodulator employing a compound phase locked loop circuit, such as a Costas loop.
A variety of communication systems has hitherto been known for transmitting data signals. Recently, a phase modulation system, or a phase shift keying (PSK) system, transmitting data by taking advantage of phase changes, has come into widespread application.
Above all, in the communication with a moving object, a bi-phase phase shift keying (BPSK) system transmitting data using two phase changes is frequently employed. A compound phase locked loop circuit (compound PLL circuit), such as a Costas loop, is frequently employed in its demodulating section. When the carrier frequency and phase of the PLL input signals coincide with the oscillation frequency and phase of a voltage-controlled oscillator (VCO), respectively, the compound phase-locked loop circuit enters the so-called locked state to perform synchronous follow-up data demodulation.
The conventional practice for detecting the locking state has been to square an in-phase I-channel component and an quadrature Q-channel component for detecting the locked state, resulting in an increased circuit scale of the locking detector and complicated calculation.
On the other hand, if, with the use of the PLL type demodulator, the frequency of the carrier wave of input signals is deviated relative to the oscillation frequency of the VCO towards a higher frequency or a lower frequency by an amount equal to an integer times of one-half of the frequency of the data transmission rate, the PLL circuit enters a pseudo-locked state in which the PLL circuit is actually not locked to correct signals and yet looks as if it were locked to correct signals. With such pseudo-locked state, data cannot be read as normally.
There are a number of locking detectors in the prior art which are unable to distinguish the pseudo-locked state from the normal locked state, such that certain prior-art systems need to have a pseudo-locking detector in addition to the locking detector.